This invention relates to a heat treatment method by which the brazing of ferric articles and their carburization are simultaneously made.
As a method for joining ferric machinery articles, it is known to braze them in a specific furnace atmosphere and without employing any flux. This kind of brazing method is free from adversely affecting qualities of articles to be treated, and suitable for treating a number of articles in succession. Carburization is also known to improve anti-abrasion and anti-fatigue characteristics of outer surfaces of ferric articles by having carbon penetrated from the outer surfaces and diffused inwardly to form hardened layers.
Generally, the aforementioned kind of brazing and carburization are made separately in independent furnaces. However, an example is described in Japanese Patent Publication No. 58-3792, in which the brazing and carburization of ferric parts are made in a single furnace. In this instance, the parts are brazed in a brazing zone kept at about 1,120.degree. C., and then, transferred into a carburization zone which succeeds to the brazing zone of the same furnace and which are divided to three zonal chambers each kept at about 1,000.degree. C., 950.degree. C., and 850.degree. C. In said zonal chambers, in which a temperature of an endothermic or exothermic atmosphere gas is lowered stepwise to the above-mentioned degrees, carburization is made by adding a hydrocarbon gas to the atmosphere gas.
This method is advantageous, compared to conventional two-process methods, since it does not require to heat articles once again from the beginning for the carburization thereof, it minimizes installations, and it requires less manpower, whereby processing costs will be reduced. However, it is very difficult though not impossible to control a carburization atmosphere accurately under predetermined temperatures in each zonal chambers, or to control, in other words, gas potentials as desired in each zonal chambers, because the zonal chambers can not be exactly separated to each other. It is disadvantageous in this method that when an exothermic gas is added by hydrocarbon gas so that it can have a carbon potential of about 1.0% carbon and accordingly it can be effective to perform carburization, the chambers will be sooted.
In another example in which the brazing and carburization are made simultaneously in a single furnace, its brazing has to be made at a temperature suited to carburization, viz., 900-950.degree. C. This compels to use solders made from Cu or Ag alloys which have a low melting point, and excludes the employment of pure copper as a solder, although the pure copper is the most stable materials for brazing ferric or steel articles and inexpensive.
It shall be noted also that the aforementioned conventional carburization methods are for making the carburization of the whole parts of articles, and that they do not aim to carburize selected parts only of the articles. As explained above, it is possible only with much disadvantages such as the occurrence of soot to control an exothermic gas so that it can have a carbon potential of about 1.0% carbon or to control an endothermic gas so that it can have a carbon potential of 0.9% carbon, if carburization is to be made at a brazing temperature suitable to the use of copper solders, viz., 1,120-1,130.degree. C. The brazing is, therefore, conducted at said temperature range of 1,120-1,130.degree. C. first and separately from the carburization. Then, articles which have been brazed, are transferred to another chamber or furnace, whereby the articles will have been cooled. The articles are reheated to have a temperature suited for the carburization, that is, about 900-1,000.degree. C., at which temperature an atmospheric gas can readily be controlled to have a carbon potential proper to the carburization. This way of processing looks like merely training a brazing furnace and a carburizing furnace Especially when an elongated furnace with continuously moving conveyor belts is employed, it becomes more difficult to control a furnace atmosphere so that it has a constant carbon potential, because the temperature of such furnace atmosphere differs at each zones such as preheating, heating, and cooling zones, while a carbon potential is greatly dependent upon a temperature.